This invention relates to a process for thermochemical generation of acid and for thermal imaging, and to an imaging medium for use in this thermal imaging process.
Thermal imaging processes are known which use a material capable of undergoing a color change from a colorless to a colored form, from one color to another color or from a colored to a colorless form upon application of heat. For example, U.S. Pat. No. 3,723,121 discloses several thermochromic materials for laser beam recording including inorganic compounds, such as black copper (II) oxide, which decomposes to red copper (I) oxide upon heating, and organic compounds, such as polyacetylene compounds, which subsequent to treatment with ultraviolet light undergo two changes in color, first to red then to yellow, as the temperature is increased.
U.S. Pat. No. 4,720,449 describes a thermal imaging method which comprises heating imagewise a di- or triarylmethane compound possessing within its di- or triarylmethane structure an aryl group substituted in the ortho position to the meso carbon atom with a moiety ring-closed on the meso carbon atom directly through a nitrogen atom, which nitrogen atom is also bound to a group with a masked acyl substituent that undergoes fragmentation upon heating to liberate the acyl group for effecting intramolecular acylation of the nitrogen atom to form a new group in the ortho position, whereby the di- or triarylmethane compound is rendered colored in an imagewise pattern corresponding to the imagewise heating.
U.S. Pat. No. 4,602,263 and U.S. Pat. No. 4,826,976 both describe thermal imaging systems for optical recording and particularly for forming color images. This thermal imaging method relies upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visually discernible color shift from colorless to colored, from colored to colorless or from one color to another. In both references, the preferred method of producing the heat required for the irreversible unimolecular fragmentation is to include in the imaging medium an infra-red absorber which generates heat upon exposure to infra-red radiation, and then to imagewise expose the imaging medium to infra-red radiation.
All thermal imaging systems which rely upon a heat-induced color change in a single material potentially suffer from the problem that, although the color change only occurs rapidly at an elevated temperature, the color change will continue at some finite, though low rate, at lower temperatures, such as ambient temperatures at which the relevant imaging medium is normally stored prior to exposure and at which the formed images are stored after exposure. Development of slight color in the imaging medium prior to exposure results in an increased minimum optical density (D.sub.min) in the image; in other words, the white portions of the image appear less white the longer the imaging medium is stored prior to exposure. Similarly, continuing color change after exposure, especially in unexposed regions of the image where the original heat-sensitive material is not decomposed during exposure, may, over a period of years, result in increased optical density in unexposed regions and a consequent loss of contrast in the image. These problems caused by unwanted color change may be exacerbated in polychrome systems by the fact that, at storage temperatures, the rates of decomposition of the various heat-sensitive materials used to produce the various colors may differ, so that when the optical density of supposedly white or grey areas of the image changes on storage, these areas may develop a colored tint rather than remaining a neutral white or grey.
The heat-sensitive materials disclosed in the aforementioned U.S. Pat. Nos. 4,602,263 and 4,826,976 comprise single compounds the molecules of which may be regarded as having a relatively small heat-sensitive center (typically a t-butoxycarbonyl group) covalently linked to a much larger chromophore (typically a polysubstituted xanthene nucleus). There are theoretical advantages to replacing such a covalently-linked compound with a two-component system comprising a small molecule which generates acid upon heating and a larger molecule which changes color upon contact with acid. Polychrome forms of such a two component system would require only a single heat-sensitive compound. By including a small amount of base with the heat-sensitive compound and the acid-sensitive compound, small amounts of acid generated during storage of the imaging medium prior to exposure could be neutralized, thereby avoiding an increase in D.sub.min in the unexposed areas of the image. Finally, such a two-component system could contain an excess of the low molecular weight heat-sensitive compound and only the amount of the high molecular weight acid-sensitive compound needed to produce the desired maximum optical density (D.sub.max) in the image. Such a system with excess heat-sensitive compound is likely to be more sensitive than a single component system, since part of the heat-sensitive material normally remains unchanged even in areas of maximum optical density; in the two-component system, use of excess low molecular weight heat-sensitive compound can compensate for its incomplete thermal breakdown without greatly increasing the mass of material to be heated, whereas a corresponding attempt to increase the amount of heat-sensitive centers in a single component system necessarily increases the amount of the high molecular weight molecule, thereby greatly increasing the mass of material to be heated.
Heat-sensitive materials which liberate acid upon heating are known. For example, Sabongi, G. J., Chemical Triggering--Reactions of Potential Utility in Industrial Processes, Plenum Press, New York, N.Y. (1987), pages 68-72 describes thermally triggered release of carboxylic acids from esters and oxime derivatives, especially benzaldoximes and oxalic acid esters, while pages 97-101 of the same work describe photochemical release of carboxylic acids from benzyl, phenacyl, sulfenyl and benzoin esters.
U.S. Pat. No. 4,603,101 describes photoresist compositions containing a compound which photochemically generates acid. The acid-generating compounds used are onium salts.
U.S. Pat. No. 4,916,046, issued Apr. 10, 1990, on application Ser. No. 243,819, filed Sep. 13, 1988, describes a positive radiation-sensitive mixture using a monomeric silylenol ether, and a recording medium produced therefrom. This patent also contains an extensive discussion of radiation-sensitive compositions which form or eliminate an acid on irradiation. According to this patent, such radiation-sensitive compositions include diazonium, phosphonium, sulfonium and iodonium salts, generally employed in the form of their organic solvent-soluble salts, usually as deposition products with complex acids such as tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroantimonic acid and hexafluoroarsenic acid; halogen compounds, in particular triazine derivatives; oxazoles, oxadiazoles, thiazoles or 2-pyrones which contain trichloromethyI or tribromomethyl groups; aromatic compounds which contain ring-bound halogen, preferably bromine; a combination of a thiazole with 2-benzoylmethylenenaphthol; a mixture of a trihalomethyl compound with N-phenylacridone; .alpha.-halocarboxamides; and tribromomethyl phenyl sulfones.
A heat-sensitive acid generating material needs to fulfil several differing requirements. It is desirable that the material generate a strong acid, since generation of a weak acid, such as the carboxylic acids generated by some of the materials discussed above, may limit the types of acid-sensitive compound which can be used. The heat-sensitive acid generating material is desirably of low molecular weight in order to reduce the amount of material required to generate a specific amount of acid, and also to reduce the amount of energy required to heat the material to its decomposition temperature. The acid generating material should decompose rapidly when heated to its acid-forming temperature, and this temperature should not be higher than about 130.degree. C., in order to reduce the amount of energy which must be supplied to decompose the acid generating material and thus reduce the energy necessary for acid formation in a medium, and increase the sensitivity of the medium. Finally, the acid generating material must be compatible with all the other components of the imaging medium in which it is to be used, and should not pose environmental problems, such as offensive smell or severe toxicity.
It has now been found that certain squaric acid derivatives are effective as heat-sensitive acid generating materials, and that these derivatives are useful in thermal imaging.